Polymeric twin wall panels, also referred to as corrugated board, or twin wall board, hollow board, or fluted sheets, have been manufactured and used for many years. Such panels have broad application for packaging, signage, building and for other similar purposes. These products have found favor as alternatives to twin wall cardboard products or Masonite as they are more durable, are not susceptible to water damage and are generally stronger than cardboard products of similar weight. Notwithstanding the advantages of polymeric twin wall products in terms of physical performance there are limitations in using these boards as signs or as packaging due to difficulties in achieving high print quality on the polymeric surface. These problems are exacerbated when running at high line speeds.
When running at high speeds, it has been observed that the surface of the corrugated board becomes increasingly pitted, particularly along the flutes. Such pits cause imperfections in the printing, which may cause dark spots in the printing due to ink pooling in the pits or blank spots where the ink does not contact the surface of the sheet. These problems may make the corrugated boards unsuitable for printed applications. This pitting problem appears to be particularly prevalent when the corrugated board has particularly thin walls, for example having wall thicknesses of less than 0.3 mm. In such applications it is generally preferred to use resins having higher melt flow rates (for example resins having an MFR greater than 1 g/10 min). It would be desirable to have polymeric twin wall boards which can be produced at line speeds greater than 5 m/min which have smooth surfaces with little or no pitting.
In one aspect, the present invention is a composition comprising an impact copolymer and a processing aid which can be used to make such polymeric twin wall boards. The impact copolymer has a melt flow range of from 1.5 to 3.5 g/10 min and a dispersed phase content of from 5 to 30% by weight. The dispersed phase of the impact copolymer comprises a copolymer comprising units derived from ethylene and units derived from one or more alpha-olefins having from 3 to 12 carbon atoms. Preferably the dispersed phase comprises a propylene-ethylene copolymer (EPR). The dispersed phase has an ethylene content of from 30- to 70% by weight of the dispersed phase (that is, the dispersed phase comprises from 30 to 70 percent by weight of units derived from ethylene). The matrix phase of the impact copolymer is a propylene homopolymer or a random copolymer comprising units derived from propylene and a second copolymer selected from either ethylene or 1-butene wherein the units derived from the second copolymer comprise from 0 to 5% by weight of the dispersed phase.
In another aspect the present invention is a corrugated board having improved printability comprising the impact copolymer of the first aspect of the invention and a processing aid.
In another aspect the present invention is a method of making a corrugated board comprising the steps of first selecting a base resin comprising the impact copolymer of the first aspect of the invention, then adding a processing aid to the base resin in an amount of from 50 ppm to 2000 ppm, and then forming the corrugated board in an extrusion process.